Precipitation of Aluminum Containing Species in Tank Wastes
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PNNL-13881 Precipitation of Aluminum Containing Species in Tank Wastes S.V. Mattigod K.E. Parker D.T. Hobbs D.E. McCready April 2002 Prepared for the U.S. Department of Energy under Contract DE-AC06-76RL01830 PNNL-13881 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor Battelle Memorial Institute, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. 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McCready April 2002 Prepared for the U.S. Department of Energy under Contract DE-AC06-76RL01830 Pacific Northwest National Laboratory Richland, Washington 99352 Summary Aluminisilicate deposit buildup experienced during the tank waste volume-reduction process at the Savannah River Site (SRS) required an evaporator to be shut down in October 1999. The Waste Processing Technology Section of Westinghouse Savannah River Company at SRS is now collaborating with a team from Pacific Northwest National Laboratory to verify the steady-state thermodynamic stability of aluminosilicate compounds under waste tank conditions in an attempt to eliminate the deposition and clogging problems. Tests were conducted to 1) identify the insoluble aluminosilicate phase(s) and characterize the chemistry and microstructure of these phases, 2) study the kinetics of the phase formation and transformation of such aluminosilicate phases under hydrothermal conditions, and 3) verify the stability boundaries in the activity diagram of interest to the 2H Evaporator, namely the critical concentrations of silica required to form insoluble aluminosilicates. The data we obtained from tests conducted at 40°C showed that formation and persistence of crystalline phases was dependent on the initial hydroxide concentrations. The formation and persistence of zeolite A occurred only at lower hydroxide concentrations, whereas increasing hydroxide concentrations appeared to promote the formation of sodalite and cancrinite. The results showed that although zeolite A forms during initial period of reaction, due to it’s metastability, converts to more stable crystalline phases such as sodalite and cancrinite. We also observed that the rate of transformation of zeolite A increased with increasing hydroxide concentration. The data from tests conducted at 80°C revealed relatively rapid formation of sodalite and cancrinite. Although minor amounts of zeolite A were initially detected in some cases, the higher reaction temperatures seemed to promote very rapid transformation of this phase into more stable phases. Also, the higher temperature and hydroxide concentrations appeared to initiate kinetically fast crystallization of sodalite and cancrinite. More recent testing at SRS in support of the high-level waste evaporator plugging issue has shown similar trends in the formation of aluminosilicate phases. Comparison of our results with those reported above show very similar trends i.e. initial formation of an amorphous precipitates followed by a zeolite phase that transforms to sodalite which finally converts to cancrinite. Our results also show the expected trend of an increased rate of transformation of initial precipitates into denser scale- forming aluminosilicate phases (sodalite and cancrinite) at higher temperature. v Acknowledgments The authors wish to thank Dr. Jonathan Icenhower, Pacific Northwest National Laboratory, and Dr. Carol Jantzen of Westinghouse Savannah River Company for providing technical reviews, and Rosalind Schrempf, Pacific Northwest National Laboratory, for technical editing and editorial review. This work is supported by the U.S. Department of Energy’s Environmental Management Science Program. vi Contents Summary .............................................................................................................................. iii 1.0 Introduction .................................................................................................................... 1 1.1 Background............................................................................................................. 1 1.2 Possible Solution..................................................................................................... 2 1.3 Literature Review on the Formation of Aluminosilicate (Zeolite) Compounds ..... 3 2.0 Materials and Methods ................................................................................................... 4 2.1 Selection of Hydroxide Concentrations .................................................................. 4 2.2 Selection of Salt Concentrations ............................................................................. 4 2.3 Reaction Temperature and Time ............................................................................. 6 2.4 Sample Preparation Methodology ........................................................................... 6 2.5 Solid and Liquid Sample Analysis .......................................................................... 7 2.6 Turbidity and pH Measurements............................................................................. 7 3.0 Results and Discussion................................................................................................... 7 3.1 Tests Conducted at 40°C ......................................................................................... 7 3.2 Tests Conducted at 80°C ......................................................................................... 9 3.3 Turbidity and pH Measurements............................................................................. 14 3.4 Comparison of Results with Previous Testing ........................................................ 16 4.0 References ...................................................................................................................... 16 Appendix A. Semi Quantitative Method for Phase Determination from X-Ray Diffraction Data................................................................................. 19 Appendix B. Solution Composition and Solid Phase Data ................................................ 23 vii 1.0 Introduction Aluminisilicate deposit buildup experienced during the tank waste volume-reduction process at the Savannah River Site (SRS) required an evaporator to be shut down in October 1999. The Waste Processing Technology Section (WPTS) of Westinghouse Savannah River Company at SRS is now collaborating with team members from Pacific Northwest National Laboratory (PNNL) to verify the steady-state thermodynamic stability of aluminosilicate compounds under waste tank conditions in an attempt to eliminate the deposition and clogging problems. This progress report describes the history of the problem, discusses relevant literature, and describes the test results obtained so far by the WPTS/PNNL team. 1.1 Background High-level wastes (HLW) from fuel-reprocessing operations are evaporated at SRS to concentrate the waste to about 30 to 40% of its original volume before it is discharged into a holding tank. The 2H-Evaporator system at SRS consists of a feed tank (Tank 43H) that receives liquid wastes primarily from fuel-reprocessing operations and the Defense Waste Processing Facility (DWPF), the evaporator, and the concentrate receipt tank (currently Tank 38H). After evaporation, the concentrated wastes are transferred via a gravity drain line to the concentrate receipt tank. Frequently, the concentrated wastes from the concentrate receipt tank are transferred back into the evaporator feed tank for further volume evaporation. For about four decades, SRS evaporators operated successfully with only occasional minor problems such as NaNO3 salt buildup and clogging of the drain lines from the evaporators. Because these deposits were water-soluble, the drain lines were unclogged easily by flushing with water. In 1997, the 2H-Evaporator feed tank began receiving silicon-rich wastes from the DWPF recycle stream. The DWPF recycle waste stream is more dilute (contain less soluble salts) than fuel-reprocessing wastes, and thus requires a higher degree of concentration (typically 90%) to reach the same salt concentrations as are found in the fuel-reprocessing wastes. The higher concentration requirement for DWPF waste and the existing operational problems resulted in significant increases in the residence time of these wastes in the 2H Evaporator. Beginning in 1997, the silicon-rich DWPF waste stream was mixed with the aluminum-rich stream from the fuel-reprocessing operation in Tank 43H, and this mixture was fed to the 2H- Evaporator.